This invention relates to a semiconductor integrated circuit device. More specifically, the invention relates to technology for preventing malfunction caused by high-frequency noise that enters through input terminals of a differential amplifier circuit, such as technology effective in coping with electromagnetic wave noise in the operational amplifier IC.
A variety of circuit forms have heretofore been proposed using a differential amplifier circuit as an operational amplifier or a comparator IC for detecting the levels of analog input signals. However, systems using the semiconductor integrated circuit device are accompanied by a problem of malfunction caused by electromagnetic interference waves. On the other hand, the operational amplifier IC and the comparator IC have generally been considered to be free from the problem of malfunction that stems from the electromagnetic wave noise owing to the employment of a differential amplifier circuit that is less affected by noise of the same phase.
In recent years, however, it has been pointed out that even the operational amplifier IC and the comparator IC are subject to malfunction due to the infiltration of electromagnetic wave noise through input terminals. In order to prevent malfunction caused by electromagnetic wave noise, therefore, there has been proposed an invention according to which a filter unit which is a capacitor using an insulating film as a dielectric is formed between an input pin and a differential amplifier circuit (Japanese Patent Laid-Open No. 167827/1997).
The above proposed invention, however, simply discloses forming the filter unit which is a capacitor using an insulating film as a dielectric between the input pin and the differential amplifier circuit, but teaches none of a concrete capacity of the capacitor or a cut-off frequency of the filter unit constituted by the capacitor.
The present inventors have analyzed the causes of malfunction of the operational amplifier IC due to electromagnetic wave noise, quite independently of the above proposed invention. As a result, the inventors have discovered the occurrence of malfunction due to the mechanism described below.
First, the inventors have speculated that the cause is due to the input of different noises to an inverted input terminal and to a noninverted input terminal, since the differential amplifier circuit is immune to the noises of the same phase that enter through the inverted input terminal and the noninverted input terminal and does not malfunction. The operational amplifier IC is used in a state where an analog signal is input to one input terminal and a reference voltage is applied to the other input terminal. In this case, the lengths of wirings are different up to the inverted input terminal and up to the noninverted input terminal, and the electromagnetic wave noises do not enter under quite the same condition; i.e., noises are often out of the same phase.
As shown in FIG. 1, therefore, a circuit is formed in a manner that ground potential is applied to an inverted input terminal (xe2x88x92) of the operational amplifier OP through a resistor r1, an end of a feedback resistor r2 is connected thereto, and ground potential is applied to a noninverted input terminal (+) through resistors r3 and r4 connected in parallel. The resistors r1 and r3 are 51xcexa9, and the resistors r2 and r4 are 5.1 kxcexa9. The resistors r3 and r4 are connected in parallel to the noninverted input terminal (+), from such a standpoint that an input offset will not occur in the circuit under the same condition as the inverted input terminal (xe2x88x92) to which the two resistors r1 and r2 are connected. The cut-off frequency fc of the operational amplifier that is used is about 300 kHz.
In this circuit, first, a high-frequency noise source RF is connected to the noninverted input terminal (+) to give it false electromagnetic wave noise, high-frequency waves are input to the noninverted input terminal (+) from the high-frequency noise source RF, and an output voltage is observed while changing the frequency. Next, referring to FIG. 2, a high-frequency noise source RF is connected to the inverted input terminal (xe2x88x92) of the operational amplifier OP to give it false electromagnetic wave noise, high-frequency waves are input to the inverted input terminal (xe2x88x92), and the output voltage is observed while changing the frequency.
FIG. 3 illustrates a change in the output voltage that is observed when high-frequency noise is input to the noninverted input terminal (+), and FIG. 4 illustrates a change in the output voltage that is observed when high-frequency noise is input to the inverted input terminal (xe2x88x92). It is learned from FIG. 3 that when high-frequency noise is input to the noninverted input terminal (+), the output voltage Vout starts decreasing from around 1 MHz which is slightly higher than the cut-off frequency fc of the operational amplifier, becomes the lowest around 100 MHz, rises thereafter and returns to the initial level around 1 GHz. It is similarly learned from FIG. 4 that when high-frequency noise is input to the inverted input terminal (xe2x88x92), the output voltage Vout starts increasing from around 1 MHz which is slightly higher than the cut-off frequency fc of the operational amplifier, becomes the highest around 100 MHz, decreases thereafter and returns to the initial level around 1 GHz.
The present inventors have studied the cause of temporary increase or decrease of the output voltage Vout over a given frequency band, and have reached the conclusion that the phenomenon mentioned below is a cause.
FIG. 5 illustrates a circuit constitution of the operational amplifier OP used in the above experiment. In this operational amplifier, level shift circuits 12 and 13 constituted by emitter followers for broadening the lower-limit level of a dynamic range of input signals toward the lower side of the ground potential, are inserted in a stage preceding an active load-type differential amplifier stage 11.
FIG. 6 shows measurement of changes in a potential V1 at an input node and in a potential V2 at an output node n2 of the level shift circuit 12 at the time when a signal of a frequency lower than the cut-off frequency fc is input to the inverted input terminal (xe2x88x92) of the operational amplifier. In this case, as will be obvious from FIG. 6, the two potentials V1 and V2 change in the same manner being deviated by a forward voltage Vbe (about 0.7 V) across base and emitter of an input transistor Q1.
FIG. 7 illustrates changes in the potential V1 at the input node and in the potential V2 at the output node n2 of the level shift circuit 12 at the time when a signal of a frequency of about 100 MHz which is higher than the cut-off frequency fc is input to the inverted input terminal (xe2x88x92) of the operational amplifier. In this case, the input potential V1 varies depending upon the input, but the potential V2 at the node n2 assumes a saw-tooth wave form of a small amplitude as shown in FIG. 7, and an average DC level is considerably lower than that of FIG. 6.
As described above, the potential V2 at the node n2 assumes the saw-tooth wave form probably because a parasitic capacity Cjs between the base and the substrate of a differential transistor Q3 in a differential amplifier stage is connected to the output node n2 of the level shift circuit and, hence, a current of a current source I1 in the level shift circuit is consumed for charging the parasitic capacity Cjs when V2 increases, and the electric charge in the parasitic capacity Cjs is quickly extracted by a collector current of the input transistor Q1 when V2 decreases. It was found that when the high-frequency wave is input to the inverted input terminal (xe2x88x92) of the operational amplifier of FIG. 5 and the DC level of the potential V2 at the node n2 decreases, the DC level varies depending upon the frequency and amplitude of the input signal.
In the system using the above operational amplifier, however, wirings of different lengths are in many cases connected to the inverted input terminal and to the noninverted input terminal as described earlier. Accordingly, high-frequency noise due to electromagnetic waves enters in different amounts into the inverted input terminal and into the noninverted input terminal, as a matter of course, and a difference occurs between a DC level Vdc2 at the node n2 in the emitter of the transistor Q1 and a DC level Vdc2xe2x80x2 at the node n2xe2x80x2 in the emitter of the transistor Q2. As a result, it is considered that an offset occurs between the inverted input terminal (xe2x88x92) and the noninverted input terminal (+) as described above, causing the operational amplifier to malfunction.
The above hypothesis is not capable of explaining the phenomenon in that the output voltage returns to the initial value when the frequency of high-frequency noise entering through the input terminal becomes higher than a certain degree as shown in FIGS. 3 and 4 (higher than 100 MHz in these drawings). This phenomenon, however, can be explained in such a fashion that the high-frequency components are attenuated by a parasitic filter circuit that is constituted by a base resistance of the transistor Q1 itself connected to the input pin of the operational amplifier and by a parasitic capacity Cjs between the base and the substrate. Thus, the inventors have reached the conclusion that a differential amplifier circuit can be realized suppressing a change in the offset caused by high-frequency noise such as of electromagnetic waves and suppressing malfunction by cutting noise having frequencies higher than the cut-off frequency of the differential amplifier circuit but lower than the cut-off frequency of the parasitic filter circuit in the input unit.
It is therefore an object of this invention to provide a differential amplifier circuit of which the offset is little likely to be changed by high-frequency noise and a semiconductor integrated circuit device that includes the differential amplifier circuit.
Another object of this invention is to provide a differential amplifier circuit that is little likely to malfunction despite it has received electromagnetic interference waves and a semiconductor integrated circuit device that includes the differential amplifier circuit.
The above and other objects as well as novel features of the invention will become obvious from the description of the specification and the accompanying drawings.
Briefly described below are representative examples of the invention disclosed in this application.
That is, between the input terminal of the differential amplifier circuit and the input node of the differential amplifier stage, there is inserted a filter circuit for cutting high-frequency noise having a cut-off frequency higher than the cut-off frequency of the differential amplifier circuit but is lower than the cut-off frequency of the parasitic filter circuit that is constituted by a parasitic capacity and a parasitic resistance in the input node.
According to the above-mentioned means, the filter circuit for cutting high-frequency noise prevents high-frequency noise such as of electromagnetic waves infiltrated through the input terminals from being transmitted to the differential amplifier stage, and suppresses a change in the input offset caused by a difference in the DC level between the inverted input terminal (xe2x88x92) and the noninverted input terminal (+) triggered by the infiltration of high-frequency noises of different amplitudes.
The filter circuit for cutting the high-frequency noise can be constituted by a CR circuit that includes a resistor and a capacitor. The capacitor can be formed by positively utilizing the parasitic capacity of the transistor to which is connected one end of the resistor. Or, a capacitor element may be formed by using, as a dielectric, an insulating film formed on the semiconductor substrate, or the capacitor element may be formed by utilizing a PN junction formed on the surface of the semiconductor substrate.
When the parasitic capacity of the transistor is positively utilized to constitute the filter circuit, a relatively small area is occupied by the filter circuit. This is effective in forming a filter circuit between the level shift circuit and the differential amplifier stage in a differential amplifier circuit having a level shift circuit in a stage preceding the differential amplifier stage. This is because, there generally exists a relatively large margin near the external input terminal for laying out the elements. On the other hand, when the insulating film is used as a dielectric to form a capacitor that constitutes the filter circuit, the cut-off frequency of the filter circuit varies little depending upon the input DC voltage since the capacity varies little depending upon the voltage compared with the junction capacity.
The resistor constituting the filter circuit may be the one that utilizes parasitic resistance by forming the base region of the transistor to which the filter circuit is connected to be larger than the base regions of other transistors. Or, a semiconductor region such as of a P-type or N-type diffusion layer formed in the surface of the semiconductor substrate separately from the base region of the transistor, may be used as the resistor, or a metal layer such as a polysilicon layer may be formed on the semiconductor substrate and may be used as the resistor. When the resistor constituting the filter circuit is formed by utilizing the parasitic resistance in the base region of the transistor, the filter circuit occupies an area smaller than that of when the resistor is separately provided. In this case, the parasitic capacity of the transistor constituting the filter circuit increases, too.
It is desired that a cut-off frequency of the filter circuit for cutting the high-frequency noise is higher than a unity gain frequency of the differential amplifier circuit but is lower than a cut-off frequency of a parasitic filter circuit in the input unit. This is because the circuit easily oscillates when the cut-off frequency of the filter circuit for cutting the high-frequency noise is set to be smaller than the unity gain frequency of the differential amplifier circuit.
A diode for electrostatic protection and a filter circuit for cutting high-frequency noise may be inserted between an input terminal of the differential amplifier circuit and an input node of a differential amplifier stage. This suppresses a change in the input offset caused by the infiltration of a high-frequency noise such as electromagnetic waves through the input terminals, and enhances the electrostatic breakdown strength owing to the protection diode.
If considered from a different point of view, the electrostatic protection diode connected to the input terminal can be regarded as a junction capacitor. Therefore, when a PN junction capacitor is used, instead of the insulating-film capacitor, as a capacitor for constituting the filter circuit for cutting high-frequency noise, there can be contrived a circuit that is also used as a diode for electrostatic protection. Upon providing a filter circuit for cutting high-frequency noise separately from the diode for electrostatic protection, however, it is allowed to optimize the properties of the elements depending upon the applications. In this case, the high-frequency noise can be cut more favorably and the electrostatic breakdown strength can be enhanced as compared with when it is used in common. Further, even in case the diode for electrostatic protection becomes defective, the filter circuit for cutting high-frequency noise works effectively. The protection diode exhibits the function of protecting the internal circuit against not only the static electricity but also the surge voltage and the surge current.